Conventionally, the filters for radiofrequency waves comprise, in addition to one or more cavities coupled together wherein dielectric resonators are arranged, means for coupling the radiofrequency energy (RF), on the one hand, to introduce RF energy at the input of the filter and, on the other hand, to extract RF energy at the output of the filter. Furthermore, they generally comprise tuning means making it possible to adjust the frequency of the main resonant modes of the filter.
Filters known from the prior art are described for example in U.S. Pat. No. 5,880,650. In this filter, a dielectric element consists of a planar plate in the form of a parallelogram, and as much as possible of the electrical field is situated in the dielectric element, which thus acts as resonator.
One advantage of the filter described in U.S. Pat. No. 5,880,650 is that the dielectric resonator is in mechanical and electrical contact with the walls of the metallic cavity by the four vertices of the plate. The vertices are truncated or rounded so as to closely follow the form of the side walls, planar or slightly incurved depending on the form of the cavity. The mechanical contact allows for an exact and reproducible positioning of the resonant element in the cavity, and heat transfer between the resonator element and the walls is significantly improved.
One drawback with this filter exists in that, because of the location of the electrical field in the dielectric element, the dielectric losses are significant. Conversely, an empty resonant cavity exhibits significant metallic losses. Since the quality factor Q depends on the metallic losses and on the dielectric losses, an empty cavity or a cavity with dielectric resonator therefore each exhibits the drawback of significant losses, that is to say a non-optimal quality factor.
Furthermore, the filter described in U.S. Pat. No. 5,880,650 was optimized for operation in band C (from 3 to 5 GHz). For it to operate at a higher frequency (for example in band Ku from 10 to 13 GHz), the dimensions should be divided by approximately three, which leads to a small filter, which is an advantage. However, the rise in frequency leads to a degradation of the quality factor Q.
Another type of filter is described in U.S. Pat. No. 8,031,036. This filter comprises a cylindrical metallic cavity and, inside, a dielectric element, also cylindrical, comprising a collar, fixed to the walls of the cavity over its entire circumference by the collar through, for example, a ring or springs. In this filter, the electrical field is concentrated in the dielectric resonator with the abovementioned drawbacks. Furthermore, the volume of the resonator cylinder is significant, leading to a heavy filter, which constitutes a notable drawback for components intended to be installed on a satellite.
One aim of the present invention is to remedy the abovementioned drawbacks.